1. Field of the Invention
The present invention relates generally to Global Navigation Satellite Systems (GNSS) receivers and, more particularly, to methods employed by the GNSS receivers to determine relative yaw.
2. Background Information
Passive ranging systems, such as Global Navigation Satellite Systems (GNSS), employ satellites to provide signals that enable a GNSS receiver to determine its position, e.g., its latitude and longitude. If a sufficient number of GNSS satellites are in the view of the receiver, the receiver also uses the signals to determine height or altitude.
The GNSS receivers utilize codes and data that are contained in the GNSS signals to determine position. A GPS receiver, for example, determines its position using the PRN codes and data that are broadcast on the L1 frequency and the PRN codes that are broadcast on the L2 frequency, with each satellite transmitting different L1 and L2 codes. The GPS receiver, operating in a known manner, synchronizes locally generated PRN codes to the received PRN codes and calculates the times it took for the transmitted codes to reach the receiver from the respective satellites. The GPS receiver then uses the travel times in conjunction with information contained in the transmitted data (e.g., satellite locations, clock information, ionospheric delay modeling information) to determine latitude, longitude and, as appropriate, altitude. Generally, the receiver requires the signals from three GPS satellites to determine position and the signals from four GPS satellites to determine altitude.
GPS signals may also be used in a craft (e.g., an aircraft) to determine the pitch, roll and yaw, or heading, of the craft. Here, pitch is defined as rotation around an “x” axis, roll as rotation around a “y” axis and yaw as rotation around a “z” axis. A prior technique that may be used to measure one axis attitude, using a GPS system is described in T. Ford et al., “Beeline RT20—a Compact, Medium Precision Positioning System with an Attitude;” Proceedings, 1997 Institute of Navigation Conference 1997, Kansas City, Mo. The technique uses two GPS antennas that are spaced apart by a predetermined distance and calculates the one axis attitude based on a known or determined baseline between one of the antennas and a base station. Using the attitude, the system then calculates yaw. Relative yaw may be described as a rate of change in the yaw or heading. The above-described technique for determining yaw may be modified to determine relative yaw by determining an initial yaw measurement at a time t1 and determining a successive yaw measurement at a later time t2 and determining the difference between the two measurements.
One problem with determining relative yaw using the above-described technique is that the system requires the circuitry necessary to determine yaw, namely, the two antennas and the corresponding sets of GPS receive circuitry to accommodate the antennas. Further, the system must make the relatively complex calculations required to compute the three axis attitude. Further, the receiver requires the signals from at least four satellites to determine the yaw and the associated relative yaw. Thus, the receiver may be precluded from determining yaw in various environments.